Overview

Arterial O2 content≈20 mL/dL

P50 (adult Hb)≈26–27 mmHg

O2 delivery / consumption≈1000 / 250 mL/min

O2 bound to Hb1.34 mL per g (fully saturated)

Each haemoglobin molecule (two α and two β globins, each with a haem containing Fe²⁺) binds up to four O2 molecules. Binding is cooperative: as O2 binds, Hb shifts from the low-affinity tense (T) state to the high-affinity relaxed (R) state, giving the sigmoid curve. O2 content CaO2 = (1.34 × [Hb] × SaO2) + (0.003 × PaO2) ≈ 20 mL/dL in arterial and ≈15 mL/dL in mixed venous blood. O2 delivery = cardiac output × CaO2 ≈ 1000 mL/min; consumption ≈250 mL/min, so ≈25% is extracted at rest and the curve's steep portion provides a large reserve.

Landmarks: PO2 100 mmHg → SaO2 ≈97–98%; 60 mmHg → ≈90%; 40 mmHg → ≈75% (mixed venous); 27 mmHg → 50% (P50). The flat upper part protects loading despite falls in PAO2; the steep lower part allows large unloading for small PO2 drops in tissues.

Right shift (↑P50, ↓affinity, favours unloading): ↑PCO2 and ↑H+ (Bohr effect), ↑temperature, ↑2,3-BPG (chronic hypoxia, altitude, anaemia), exercise. Left shift (↓P50, ↑affinity): ↓PCO2/alkalosis, hypothermia, ↓2,3-BPG (stored blood), fetal haemoglobin (γ chains bind 2,3-BPG poorly; P50 ≈19 mmHg — favours transfer across the placenta), carbon monoxide, methaemoglobin. Myoglobin has a hyperbolic curve far to the left (P50 ≈2–3 mmHg), so it releases O2 only at very low PO2 in muscle.

Location: Erythrocytes in the blood.

Function

Loads O2 in the lungs and delivers it to tissues according to metabolic need.

Clinical significance

Anaemia vs hypoxaemia

In anaemia PaO2 and SaO2 are normal but O2 content and delivery are low. Cyanosis appears when capillary deoxyhaemoglobin exceeds ≈50 g/L, so it may be absent in severe anaemia and prominent in polycythaemia.

Carbon monoxide poisoning

CO binds Hb ≈200–250 times more avidly than O2, lowering content and shifting the curve left; PaO2 and standard pulse oximetry are falsely reassuring. Headache, confusion, "cherry-red" skin (late). Treat with 100% O2 (half-life of COHb ≈4–5 h on air, ≈1 h on 100% O2) or hyperbaric O2.

Methaemoglobinaemia

Oxidation of haem iron to Fe³⁺ (nitrates, dapsone, local anaesthetics such as benzocaine, congenital) prevents O2 binding and left-shifts the remaining Hb: chocolate-brown blood, SpO2 stuck around 85%, normal PaO2. Treat with methylene blue.

Mnemonics

Right shift

"CADET, face Right" — CO2, Acid, 2,3-DPG, Exercise, Temperature.

Compiled from standard anatomy and physiology references — see the Respiratory System sources. For education only; not medical advice.